There has been proposed a brushless motor drive circuit of the type supplying a power to the brushless motor by applying to the driving coils of motor driving current waveform with dulled or rounded inflection points, in the respective phases, in an attempt to reduce a noise generation caused by abrupt or acute changes in driving current waveform due to the inherit abrupt power-supply switching in each driving coil. However, with such drive circuit, when a power transistor becomes saturated upon starting a motor or during its uncontrolled drive, the switching waveform (i.e. the driving current waveform applied to each driving coil) is distorted or becomes unsmooth, resulting in lowering its motor noise reduction effect.
One possible technique that has been proposed to solve this problem is a technique for detecting the state of the power transistor which is about to be saturated and limiting a control current so as to limit power supply to the transistor, thereby preventing saturation. This technique is disclosed in Japanese Patent Laid-Open Gazette No. 56188/88 of a patent application filed in the name of the applicant of the present application. Referring now to FIG. 6, a brief explanation will be made thereof.
In FIG. 6, the power supply to three-phase driving coils L.sub.1, L.sub.2, and L.sub.3 is controlled by power transistors Qa, Qb, Qc, Qd, Qe and Qf via a pre-driver 2. The pre-driver 2 combines together or synthesizes rotor position sensed signals from position sensors (not shown) into a 120.degree. switching signal which supplies power for a 120.degree. period in a 180.degree. interval or zone, thereby dulling or rounding this signal waveform at its inflection points to obtain a soft switching waveform, which is used to control each of the transistors Qa to Qf, effecting a power supply control for each of the driving coils L.sub.1 to L.sub.3.
The current to each of the driving coils L.sub.1 to L.sub.3 flows into a current detecting resistor Re, which generates a voltage proportional to the incoming current, and the voltage is applied to an inverting input of a current feedback amplifier Af. On the other hand, when a current flows to a resistor Ra via a transistor Qg, a voltage proportional to this current is induced in the resistor Ra and is applied to a non-inverting input of the amplifier Af. A control amplifier Ac amplifies a difference between a control reference voltage Vref, defined by the ratio between resistors Rc and Rd, and a control signal voltage Vctl, to provide a control current Ictl. The control current Ictl flows to the ground via a resistor Rb and a transistor Qj and, at the same time, defines a current which flows in the collector of a transistor Qh via the base of a transistor Qi, and in accordance with this collector current, the base current of a transistor Qg is controlled to thereby determine the above-mentioned current Ictl which is the collector current of the transistor Qg. The control current Ictl of the control amplifier Ac is set such that it will ultimately become equal to the current Ictl which flows in the resistor Ra.
Diodes Da, Db and Dc have their cathodes connected to the driving coils L.sub.1, L.sub.2 and L.sub.3, respectively, and have their anodes all connected to an output terminal of the control amplifier Ac.
Now, let is be assumed that the motor is placed under constant speed control. If the rotational speed of the motor decreases, the control voltage Vctl will be raised to increase the speed and also the control current Ictl will go up, causing an increase in the voltage of the resistor Ra. When the voltage of the resistor Ra becomes higher than that of resistor Re, the current feedback amplifier Af will, via the pre-driver 2, raise the base currents of the transistors Qa to Qf to enlarge the amplitudes of the voltages which are applied to the driving coils L.sub.1, L.sub.2 and L.sub.3, thereby making higher the coil driving currents for speed-up of the motor. In the event of the motor speed increasing excessively, it is decreased by reversing the above-said procedure. In this way, the rotational speed of the motor is held at a fixed value.
Next, a description will be given of operations at the start of the motor and during its uncontrolled running. When the control voltage Vctl increases and the control current Ictl also increases correspondingly at the start of the motor and during its uncontrolled running, the amplitude of the coil driving voltage increases and the minimum voltage of each driving coil drops. If the minimum voltage of the coil drops down to a minimum voltage value, or, Rb.multidot.Ictl, a portion of the control current Ictl flows in any one of the transistors Qd, Qe and Qf via the diode Da, Db or Dc. In consequence, the increase of the control current Ictl is not transmitted to the current feedback amplifier Af and the minimum voltage of the coil will no longer fall.
As shown in FIG. 7, the driving coil voltage varies up and down by the same amplitude, about Vcc/2. If maximum and minimum values of the driving coil voltage are sufficiently different from a DC power supply voltage Vcc and the ground potential GND, respectively, the waveform of the driving coil voltage will become a 120.degree. soft switching waveform having dulled inflection points. If, however, the maximum and minimum values of the driving coil voltage are close to the DC power supply voltage Vcc and the ground potential GND, the waveform of the driving coil voltage will become a 180.degree. soft switching waveform as indicated by the broken lines in FIG. 7, which causes vibration in the driving coil and also impairs the motor efficiency. Yet, according to the prior art example depicted in FIG. 6, the coil voltage will not reach the power supply voltage Vcc and the ground potential GND and the power transistors Qa through Qf will operate unsaturated; hence, it is possible to attain the intended object of preventing saturation.
In general, however, the saturation voltage of a transistor varies with its collector current and ambient temperature and is influenced by the rating of the transistor itself as well, and accordingly it is very difficult to accurately detect the saturation of the transistor under whatever conditions. According to the prior art example shown in FIG. 6, since terminal voltages of the diodes Da, Db and Dc are uniformly used to detect the saturation of the power transistors Qa to Qf, such situation occasionally arises where the transistors are not sufficiently kept from saturation or the saturation preventing effect is produced so much that the power supply cannot efficiently be utilized. Moreover, since the saturation preventing effect is influenced by the value of the resistor Re for motor current detection use as well, the value of the resistor Re must be fixed, hence excluding any desired designability in the circuit. Besides, spikes in the voltage waveform, which are produced when the power supply is switched to each coil, cause malfunction of the saturation preventive circuit and cut off the motor current momentarily, resulting in lower torque and speed of the motor and a noisy motor operation.